Eukaryotic cells contain complex networks of signal transduction proteins, allowing them to respond to diverse environmental cues. Given the large number of signaling pathways in a cell, a major question is how efficient and specific signaling connections are made. An emerging paradigm is that scaffold proteins or other organizing factors play an important role in maintaining signaling specificity. Scaffold proteins interact with multiple proteins in a pathway and are hypothesized to act as the "wiring" that physically organizes these proteins, promoting highly specific signaling. Scaffold proteins are found in diverse organisms, cell types and pathways. Despite the importance of scaffold proteins, little is known about the mechanisms by which they control cellular information flow. Our overall goal is to understand the molecular basis by which scaffolds control signaling efficiency and specificity. We propose to undertake a quantitative biophysical, kinetic, and structural analysis of two model scaffold proteins involved in yeast mitogen activated protein (MAP) kinase signaling: Ste5, a scaffold for the mating response pathway, and Pbs2, a scaffold for the osmolarity response pathway. These pathways have been well-studied genetically, but only recently has our lab had success purifying these proteins to allow their thorough biophysical characterization. Our specific aims are to: 1. Determine the structural organization of the mating MAPK complex organized by the scaffold protein Ste5 using crystallographic, mutational, and chemical crosslinking methods. 2. Elucidate the biochemical mechanism by which the mating scaffold Ste5 actively promotes signal transmission from Ste7 (MAPKK) to Fus3 (MAPK). 3. Reconstitute and structurally characterize the yeast osmolarity response MAPK pathway organized by the Pbs2 scaffold protein. 4. Explore the use of the Ste5 scaffold protein as a platform for pathway engineering: tuning pathway response dynamics via artificial recruitment of positive and negative effectors.